The present invention is directed to a rotor hub fairing system for a rotary-wing aircraft having a counter-rotating, coaxial rotor system, and more particularly, to a de-rotation system which rotationally stabilizes a shaft fairing mounted between an upper hub fairing and a lower hub fairing.
Typically, the aerodynamic drag associated with a rotor hub on a rotary wing aircraft is a significant portion of the overall aircraft drag, typically 25% to 30% for conventional single-rotor helicopters. The rotor system drag increases for a rotary wing aircraft having a counter-rotating, coaxial rotor system primarily due to the dual rotor hubs and the interconnecting shaft therebetween. For high-speed rotary wing aircraft, the increased drag resulting from the counter-rotating, coaxial rotor system may result in a relatively significant power penalty.
The aerodynamic drag of the dual counter-rotating, coaxial rotor system is generated by three main components—the upper rotor hub assembly, the lower rotor hub assembly, and the interconnecting main rotor shaft assembly. The drag contribution may be approximately 40% for each of the hubs, and 20% for the interconnecting main rotor shaft assembly. Typically, a rotor hub fairing arrangement is mounted to each of the upper rotor hub and the lower rotor hub such that overall drag on the rotorcraft is reduced. The interconnecting main rotor shaft between the upper rotor hub assembly and the lower rotor hub assembly, however, is typically exposed.
For a variety of reasons including, but not limited to, reduced drag and low observability, contoured shaft fairings have been developed to cover the exposed interconnecting main rotor shaft.
The shaft fairing is mounted to the counter-rotating, coaxial rotor system within a completely rotational environment between the upper hub fairing and the lower hub fairing through a bearing arrangement such that the shaft fairing is aligned with the fuselage in forward flight but is free to align with the relative wind during low speed maneuvering.
During forward flight, airflow stabilizes the shaft fairing on the bearing arrangement relative the rotating interconnecting main rotor shaft. During hover and low speed maneuvers, however, frictional forces acting on the bearing arrangement may tend to cause the shaft fairing to undesirably rotate in unison with the main rotor system. Rotation of the shaft fairing may increase drag and reduce the low-observability benefits of the contoured shaft fairing.
Accordingly, it is desirable to provide a de-rotation system for a shaft fairing within the completely rotational environment between an upper rotor hub fairing and a lower rotor hub fairing of a counter-rotating, coaxial rotor hub fairing system.